With such a known method, particles are introduced into the flow to be investigated, the flow is therefore seeded. With the correct choice of the particles, especially of their diameter, the former largely follow the flow. If therefore, the path of a particle is traced, this yields both the velocity and also the direction and orientation of the flow vector.
Such a method of the type mentioned at the beginning is known from the article "Particle Imaging Velocimetry in a Low Turbulent Wind Tunnel and other Flow Facilities" by J. Kompenhans and J. Reichmuth, AGARD Conference Proceedings No. 399, "Advanced Instrumentation for Aero Engine Components". The particles are illuminated in a plane by means of a light beam, the illumination taking place in the form of a light flash. After a pre-determined time interval, a renewed illumination of the particles takes place. The light reflected by the particles is retained on an image.
In this connection, the image is illuminated twice by the reflected light of the first and second light flash, so that each particle is imaged twice on the one image. To determine the velocity, it is now necessary to determine the images of each particle which belong together. This is achieved in that the image developed in the form of a slide is transilluminated by the second coherent light beam. This light beam is focused in the Fourier plane. Consequently, the first Fourier-transformed image is imaged in the Fourier plane. This first Fourier-transformed image has a strip pattern, the separation of the strips being inversely proportional to the velocity. The velocity vector is perpendicular to the strips. The further evaluation is undertaken by a computer, to which is fed the first Fourier-transformed image. The second Fourier-transformed image is computed in the computer. This second Fourier-transformed image has three points, the connecting line between the middle point and one of the two other points yielding the flow vector. After these steps have been carried out, the flow vector is then known at one point of the flow field. It is disadvantageous that this method is very time-consuming: The light reflected by the particles is retained on a film, which must first be developed, in order to be able to process the flow information further. Only after this first Fourier-transformed image is developed can it be fed into the computer and then evaluated by the latter. The evaluation in the computer is also very time-consuming. Normal times for the evaluation of a measured point lie in the region of about one minute. In addition to the costs arising for the film material and the development, as well as for the acquisition and maintenance of the computer, it is especially disadvantageous that a continuous monitoring of the flow is not possible. In addition, because of different development and different film material the slides are often of different quality, and are in part not suitable for the subsequent evaluation. It is then necessary to prepare a new slide, so that the total time required can be greatly lengthened. However, for some investigations, in which the flow is not reproducible, this is not possible, so that the method cannot be applied for such flow investigations. Even for the case in which the flow is reproducible, the method is not suitable for industrial measurements, because the evaluation is too tedious, and it is necessary, as a rule, to repeat some recordings, so that the possibility must exist to reproduce the flow anew after several days, the wind tunnel must therefore be available for several days. However, for reasons of cost this is not possible.